Video Transcript
The table below shows the color
range for several different indicators. Which indicator is blue at a pH of
five? Which indicator would be best to
show that the pH of a solution has changed from eight to 11?
Indicators are weak acids and bases
that undergo a color change over a specific pH range. Indicators are often used in
acid–base titrations as their color change can signify that the end point of the
titration has been reached. Different indicators change color
over different pH ranges. The table shows us the range of pH
values in which four different indicators change color. Beyond the ranges shown, the color
of the indicator will remain the same. So bromophenol blue will be yellow
below a pH of three and blue above a pH of approximately 4.6. Here, we’ve extended the colors for
each of the three remaining indicators.
To answer the first question, we
want to know which indicator is blue at a pH of five. Looking at pH five, we see that
alizarine yellow is yellow, cresolphthalein is colorless, bromothymol blue is
yellow, and bromophenol blue is blue. So the indicator that is blue at a
pH of five is bromophenol blue.
To answer the second question, we
need to select the indicator that would best show that the pH of a solution has
changed from eight to 11. This means that we should select an
indicator that undergoes a color change over this range. We can rule out bromophenol blue
and bromothymol blue, as from pH eight to 11 both indicators will remain blue. Alizarine yellow does undergo a
color change in this pH range. However, the color change doesn’t
occur until the pH gets closer to pH 11. Cresolphthalein, however, undergoes
a drastic color change from approximately pH eight to pH 10, changing from colorless
to light purple to dark purple. This means that the indicator that
would be best to show that the pH of a solution has changed from eight to 11 is
cresolphthalein.
